Significant reductions in the sound pressure levels of sound carried through confined enclosures such as ducts has been an unresolved problem for many years. In factories, for example, the noise produced by machinery and various manufacturing operations may be carried from the heating and ventilating ducts in such areas to the ducts which connect to offices and other parts of the plants in which a low level of noise is desired. This is particularly a problem with low frequency noise in the range of infrasound to 800 Hertz, since passive means to attenuate such frequencies are costly, relatively inefficient and physically large in size making them impractical for use in most low frequency applications.
Beginning in 1925 and continuing at an extremely rapid pace today, developments in electronics have made the concept of "active" attenuation of noise to be not only a possible but attractive alternative to passive attenuation of low frequencies. The principle of so-called active attenuation is based on the fact that the speed of sound in air is much less than the speed of electrical signals. In the time it takes for a sound wave to propagate from a location where it is detected to a remote location where it may be attenuated, there is sufficient time to sample the propagating wave, process such information within an electronic circuit and produce a signal to drive a speaker which introduces cancelling sound 180.degree. out of phase and equal in amplitude to the propagating sound. Although the process of active attenuation of sound stated above appears to be conceptually simple, a review of the prior art in this area will illustrate the complexity of the problem and the difficulty of obtaining good attenuation over a relatively broad band of lower frequencies.
One of the first efforts in the area of active attenuators is disclosed in U.S. Pat. No. 2,043,416 to Lueg shown in FIG. 1. The Lueg system is a monopole consisting of a microphone, amplifier and speaker. The microphone detects the source sound and converts its into an electrical signal which is fed to the amplifier. The loudspeaker, driven by the amplifier, is disposed downstream from the microphone at a location to give the necessary time delay to accomplish a 180.degree. phase reversal from the source sound. The loudspeaker injects a mirror image of the source sound into the duct so that as the source sound passes the location of the loudspeaker, a volume of either high or low pressure air is introduced 180.degree. out of phase with the corresponding high and low pressure volume of air of the source sound. When the loudspeaker is perfectly synchronized with the passage of the source noise, the pressure of the source noise and that of the loudspeaker averages to 0 (ambient static pressure) and the noise is "cancelled".
It is apparent from an examination of the Lueg system that attenuation will occur if the distance between the microphone (where the source sound is sampled) and the loudspeaker (where the cancelling sound is introduced) is such that the phase shift of the electrical signal sent to the amplifier equals 180.degree. or an odd multiple of 180.degree.. However, this condition will occur only for a specific stationary acoustic signal which does not vary in time. As a practical matter therefore, the Lueg system is effective only for a single frequency since no means are provided to accommodate phase change. What this limitation of the Lueg system shows is that there are two parameters which must be met for good attenuation, including delay time, to allow the acoustic wave to move from the point of detection to the point of attenuation, and phase, to assure that attenuation occurs at the point of introduction of the cancelling acoustic wave.
In addition to the limitation of the Lueg system associated with phase detection and accomodation, a problem exists with the generation of standing waves by the loudspeaker in the upstream direction toward the microphone. Because of the standing wave pattern, the pressure of the sound field at the microphone is artificially nonuniform which means that at a given frequency the microphone may be located at a node or antinode of a standing wave. Therefore the cancellation signal produced by the speaker may be made to be too great or too little. As a result, the sound field may be amplified by the standing waves in such a way that the resulting propagation downstream from the speaker could be even greater than the sound produced by the source. In addition, the standing wave field could intensify the sound pressure in the duct and more sound could pass through the walls of the duct creating a secondary problem.
In an effort to avoid the above-identified standing wave problem and expand the frequency range of attenuation, several active attenuation systems have been developed subsequent to the Lueg system. One prior art system, shown in FIG. 2, utilizes the combination of a monopole/dipole arrangement with the dipole being located on one wall of the duct and the monopole being located on the opposite side of the duct as shown. This system was first introduced by M. Jessel and G. Mangiante and is described in a paper entitled, "Active Sound Absorbers In An Air Duct", JSV (1972) 23 (3,383-390). The dipole and monopole of the Jessel system are phased so that they add in the downstream direction and subtract in the upstream direction allowing a unidirectional propagation to be formed when the sources are balanced. It has been found however that the results obtainable with the Jessel system are frequency dependent and related to the half wave length of the dipole separation. In addition, the complexity of this system does not lend itself for use in many practical applications.
As a means of simplification of the Jessel system and to obtain improved performance, the dipole systems shown in FIGS. 3 and 4 were developed. The system shown in FIG. 3 is discussed in U.S. Pat. No. 4,044,203 to Swinbanks. Swinbanks removed the monopole found in Jessel and altered the phase characteristics of the dipole so that the propagation of both sources is added in the downstream direction and cancelled in a direction upstream toward the microphone. The active attenuator shown in FIG. 4, and disclosed in U.S. Pat. No. 4,109,108 to Coxon et al, locates the microphone between two loudspeakers to produce a minimum level at the microphone position when the proper phasing between the speakers is introduced. While this system is reflective and produces a standing wave pattern upstream of the dipole, the detection system (microphone) is not affected because it is located between the speakers, unlike the Swinbanks system.
In reviewing the performance of the dipole and dipole/monopole systems, it has been found that each of these multisource systems seem to have geometry related limitations. The physical spacing of the loudspeakers and microphones produces a "tuning effect" which sets the frequency of best performance and the bandwidth. Although high levels of attenuation are possible, particularly with the Swinbanks systems, such a performance is obtainable only through a relatively small band width on the order of about 21/2 octaves maximum. Accordingly, the most recent approaches to active noise attenuation in ducts have concentrated on improvement of the monopole system first introduced by Lueg as discussed above. U.S. Pat. No. 4,122,303 to Chaplin et al is one example of a monopole system in which relatively sophisticated electronic circuitry is incorporated into the basic microphone/loudspeaker construction found in the Lueg system. In Chaplin et al, a secondary cancelling wave is produced by a first electrical signal which represents the primary sound wave sensed by a microphone. The first electrical signal is convolved with the second signal derived from the system impulse response as a program of operational steps. This process represents the standard operation of an adaptive filter as described in "Feedback and Control Systems", Schaum's Outline Series, 1967, pp. 179-185; "Adaptive Filters", Widrow, B., Aspects of Network and System Theory, (1971); "Principles and Applications of Adaptive Filters: A Tutorial Review", McCool, J. M.; Widrow, B., Institute of Electrical Engineers, (1976). To avoid the problem of standing waves produced by the loudspeaker and propagated upstream to the microphone, Chaplin et al suggests the use of a unidirectional microphone or lining the duct between the speaker and microphone with acoustically absorbant material. In addition, Chaplin et al suggests the possibility of incorporating circuitry to provide a second convolution capable of compensating for the feedback signal produced by the upstream standing waves. However, as discussed in more detail below, the so-called adaptation process of the Chaplin et al system occurs over a period of from 5 to 30 minutes and is thus dependent on the source signal remaining essentially constant over that period. Moreover, the control system functions so slowly that in most practical cases it must be manually shut off while the first convolution process is still operative to avoid "system hunting" or oscillation between better and poorer results.
It should also be noted that each of the systems identified above share a common problem which in many practical applications will reduce if not eliminate their potential use. In each case, the speaker portion of the system is placed directly in the environment of the duct to introduce the cancelling sound. It is anticipated that in many applications existing speakers will not be able to survive the temperatures, particulates or other foreign materials found in the environment of the duct or in unprotected areas outside of the duct.